A conventional spread spectrum signal can be viewed as the result of mixing a narrowband information-bearing signal i[t] with an informationless wideband “spreading” signal p[t]. If Bi and Bp denote the bandwidths of i[t] and p[t], respectively, then the “processing gain” available to the receiver is G=Bp/Bi. The receiver synchronizes the incoming signal to a locally generated version po[t] of p[t] and mixes the received signal with po[t], thereby removing p[t] from the signal and “collapsing” the signal to the “information bandwidth” Bi.
The spreading signal p[t] is typically a coding sequence of some kind, such as a pseudo-random code. The United States space program initially utilized a Type 1 Reed-Muller code for deep-space communications. In many code division multiple access (CL)MA) systems, the code is an M-sequence which has good “noise like” properties yet is very simple to construct.
For example, in the IS-95 standard for cellular communication, the forward channel (base to mobile units) employs, as a spreading code, the product of a 64 chip Walsh code (aimed at separating up to 64 different users per base) and a periodic PN sequence (aimed at separating the different bases). Thus, the spreading signal p[t] for each user is its Walsh code combined with the current 64 chips of the PN sequence of its base station.
In order to synchronize the local version po[t] of the spreading signal with the original version p[t], the base station additionally transmits the current PN sequence via a pilot signal z[t] (the pilot signal z[t] is simply the current PN sequence multiplied by the all 1 Walsh code). The mobile unit then synchronizes its local code generator to the pilot signal after which the mobile unit can despread the received information bearing signals using its Walsh code and the current PN sequence.
The Walsh codes Wj, I=1, . . . 64 are perfectly orthogonal to each other such that, in a non-dispersive transmission channel, there will be complete separation among the users even despite being transmitted at the same time and on the same transmission frequencies.
Practical channels, however, are time dispersive, resulting in multipath effects where the receiver picks up many echoes of the transmitted signal each having different and randomly varying delays and amplitudes. In such a scenario, the code's orthogonality is destroyed and the users are no longer separated. Consequently, a mobile unit, when attempting to detect only a single user, regards all other channel users (including signals from other base stations) as creators of interference. This contributes to a decrease in signal-to-noise ratio (SNR) and thus, reduces the reception quality of the mobile unit.
In the presence of multipath channels, the mobile units additionally process the informationless pilot signal to identify and track the multipath parameters of the channel. For this purpose, the mobile units include a channel estimator which detects and tracks the attenuation, denoted by channel “tap” ĥi, and the relative delay, denoted by {circumflex over (τ)}i, for each of the main paths. The mobile units then utilize the channel information in their detection operations.
One exemplary multipath detector is a rake receiver which optimally combines the different paths into a single replica of the transmitted signal. Rake receivers are described in detail e.g. in the book Digital Communications by J. G. Proakis, McGraw-Hill, Third Edition, 1995. The book is incorporated herein by reference.
A multiple-user detection scheme, such as is often used in base stations, can be viewed as interpreting the cross-talk between the signals of the users as merely a part of the multiple-input, multiple-output channel distortion. The base station accounts for this distortion during the detection process and, in general, the distortion does not translate into an SNR reduction. Therefore, it is not surprising that, with practical multipath channels, multi-user detection schemes are far superior to single-user ones.